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A discussion is given of the effects of a pair of pulses of rf magnetic field (at the NMR frequency) on the Zeeman and dipolar energies of the spin system in a solid. Zeeman order is partly transformed into dipolar order when the two pulses have different rf phases. A theoretical calculation provides a very simple relation between the efficiency of this transfer of energy (as a function of the characteristics of the pulses and their time separation) and the magnitude and shape of the dipolar component of the free-precession signal. This prediction has been verified quantitatively in the case of the F^19 spins in a CaF₂ crystal with the large magnetic field in a 100 direction. A maximum efficiency of transfer of Zeeman energy into dipolar energy of 56% has been obtained with a 2 phase shift between a first 2 pulse and a second 4 pulse, separated by a time of the order of T₂. (An ideal adiabatic demagnetization in the rotating frame would have an efficiency of 100%. ) The experimental results also indicate that the F^19 spin system reaches internal quasiequilibrium in a time of the order of a few T₂ after the pulse pair. It is also shown that the use of a pair of rf pulses (both of the same rf phase this time) leads to a new method for measuring the complete shape of free-precession signals, which avoids the usual difficulties due to the finite recovery time of the observation circuit.
Jeener et al. (1967) studied this question.